Phase Boundary Mapping for Exploring New Thermoelectric Zintl Compounds

Understanding and controlling the defect chemistry of bulk materials can vastly increase the opportunities for discovering highly efficient thermoelectrics. Good thermoelectrics are degenerate semiconductors and there are two types: n-type thermoelectrics, whose charge carriers are electrons, and p-type thermoelectrics conducting holes as carriers. Although normally one type can attain superior thermoelectric properties to the other depending on the electronic band structure of a material, a formation of the unfavorable defects sometimes prevents a material from obtaining the desired type. Similarly, even if the desired carrier type is realized, the Fermi level, which is a measure of the carrier density, could be kept from the optimum due to the formation of compensating defects. It has been known from growing binary semiconductors for electronics and optoelectronics such as GaAs and GaN that a growth condition can substantially alter the defect concentration of resulting samples. This is primarily due to the change in the reference atomic chemical potentials, but such defect engineering has not been utilized for the bulk thermoelectric research, resulting in overlooking the promising candidate materials. In this work, we established an experimental methodology to fully explore all the accessible variations in chemical potentials of a target phase and demonstrated its implementation. Although a pursuance of purity of samples to be measured is a common experimental concept in solid-state chemistry, in practice, a small single-phase region of semiconductors allows samples to have a certain amount of impurities. Since different multi-phase equilibria have discrete chemical potentials, only when all the boundaries of the multi-phase equilibria around target phase are mapped out in nominal composition space are the measured transport properties of resulting samples properly correlated with their atomic chemical potential. Utilizing this experimental concept we call "phase boundary mapping", we have identified the mechanism of obtaining the superior n-type conduction in Mg3Sb2-based compounds. To achieve their exceptionally high thermoelectric figure-of-merit (zT = 1.5 at 750 K), the formation energy of Mg-vacancy needs to be suppressed with excess Mg but this condition had been missing for over 80 years due to the absence of experimental concept to fully investigate properties of a material. Implementing phase boundary mapping has also allowed an inexpensive thermoelectric Zintl compound Ca9Zn4+xSb9 to be one of the best thermoelectrics in the intermediate temperature range (zT = 1.1). We have also successfully reduced the carrier concentration of Yb9Zn4+xSb9, which was originally thought to be impossible, leading to zT increased by a factor of five.</p

Thermoelectric properties of the Yb_9Mn_(4.2-x)Zn_xSb_9 solid solutions

Yb_9Mn_(4.2)Sb_9 has been shown to have extremely low thermal conductivity and a high thermoelectric figure of merit attributed to its complex crystal structure and disordered interstitial sites. Motivated by previous work which shows that isoelectronic substitution of Mn by Zn leads to higher mobility by reducing spin disorder scattering, this study investigates the thermoelectric properties of the solid solution, Yb_9Mn_(4.2−x)Zn_xSb_9 (x = 0, 1, 2, 3 and 4.2). Measurements of the Hall mobility at high temperatures (up to 1000 K) show that the mobility can be increased by more than a factor of 3 by substituting Zn into Mn sites. This increase is explained by the reduction of the valence band effective mass with increasing Zn, leading to a slightly improved thermoelectric quality factor relative to Yb_9Mn_(4.2)Sb_9. However, increasing the Zn-content also increases the p-type carrier concentration, leading to metallic behavior with low Seebeck coefficients and high electrical conductivity. Varying the filling of the interstitial site in Yb_9Zn_(4+y)Sb_9 (y = 0.2, 0.3, 0.4 and 0.5) was attempted, but the carrier concentration (~10^(21) cm^(−3) at 300 K) and Seebeck coefficients remained constant, suggesting that the phase width of Yb_9Zn_(4+y)Sb_9 is quite narrow

Observation of valence band crossing: the thermoelectric properties of CaZn_2Sb_2–CaMg_2Sb_2 solid solution

CaAl_2Si_2 type Zintl phases have long been known to be promising thermoelectric materials. Here we report for the first time on the thermoelectric properties of CaMg_2Sb_2 along with the transport properties of CaZn_2Sb_2–CaMg_2Sb_2 solid solution. The charge carrier tuning in this system was carried out by substituting divalent Ca^(2+) with monovalent Na^+. To check a possible band convergence, we applied an effective mass analysis to our samples and found an abrupt doubling of the samples' effective masses as the composition switches from Zn-rich to Mg-rich. We further analyzed the effect that alloy scattering plays in the lattice thermal conductivity of our samples with a Modified Klemens model. We showed that the reduction seen in the lattice thermal conductivity of the alloyed samples can be well explained based on the mass difference of Mg and Zn in the poly-anionic metal site. Our best p-doped sample with a composition of Ca_(.99)Na_(.01)MgZnSb_2 displays a relatively high peak zT of 0.87 at 850 K

Observation of valence band crossing: the thermoelectric properties of CaZn_2Sb_2–CaMg_2Sb_2 solid solution

CaAl_2Si_2 type Zintl phases have long been known to be promising thermoelectric materials. Here we report for the first time on the thermoelectric properties of CaMg_2Sb_2 along with the transport properties of CaZn_2Sb_2–CaMg_2Sb_2 solid solution. The charge carrier tuning in this system was carried out by substituting divalent Ca^(2+) with monovalent Na^+. To check a possible band convergence, we applied an effective mass analysis to our samples and found an abrupt doubling of the samples' effective masses as the composition switches from Zn-rich to Mg-rich. We further analyzed the effect that alloy scattering plays in the lattice thermal conductivity of our samples with a Modified Klemens model. We showed that the reduction seen in the lattice thermal conductivity of the alloyed samples can be well explained based on the mass difference of Mg and Zn in the poly-anionic metal site. Our best p-doped sample with a composition of Ca_(.99)Na_(.01)MgZnSb_2 displays a relatively high peak zT of 0.87 at 850 K

Band engineering in Mg_3Sb_2 by alloying with Mg_3Bi_2 for enhanced thermoelectric performance

Mg_3Sb_2–Mg_3Bi_2 alloys show excellent thermoelectric properties. The benefit of alloying has been attributed to the reduction in lattice thermal conductivity. However, Mg_3Bi_2-alloying may also be expected to significantly change the electronic structure. By comparatively modeling the transport properties of n- and p-type Mg_3Sb_2–Mg_3Bi_2 and also Mg_3Bi_2-alloyed and non-alloyed samples, we elucidate the origin of the highest zT composition where electronic properties account for about 50% of the improvement. We find that Mg_3Bi_2 alloying increases the weighted mobility while reducing the band gap. The reduced band gap is found not to compromise the thermoelectric performance for a small amount of Mg_3Bi_2 because the peak zT in unalloyed Mg_3Sb_2 is at a temperature higher than the stable range for the material. By quantifying the electronic influence of Mg_3Bi_2 alloying, we model the optimum Mg_3Bi_2 content for thermoelectrics to be in the range of 20–30%, consistent with the most commonly reported composition Mg_3Sb_(1.5)Bi_(0.5)

Improving the thermoelectric performance in Mg_(3+x)Sb_(1.5)Bi_(0.49)Te_(0.01) by reducing excess Mg

The thermoelectric performance of Mg_(3+x)Sb_(1.5)Bi_(0.49)Te_(0.01) was improved by reducing the amount of excess Mg (x = 0.01-0.2). A 20% reduction in effective lattice thermal conductivity at 600 K was observed by decreasing the nominal xfrom 0.2 to 0.01 in Mg_(3+x)Sb_(1.5)Bi_(0.49)Te_(0.01), leading to a 20% improvement in the figure-of-merit zT. Since materials with different amounts of Mg have similar electronic properties, the enhancement is attributed primarily to the reduction in thermal conductivity. It is known that excess Mg is required to make n-type Mg_(3+x)Sb_(1.5)Bi_(0.49)Te_(0.01); however, too much excess Mg in the material increases the thermal conductivity and is therefore detrimental for the overall thermoelectric performance of the material

High-Temperature Thermoelectric Properties of the Solid–Solution Zintl Phase Eu11Cd6Sb12–xAsx (x \u3c 3)

Zintl phases are compounds that have shown promise for thermoelectric applications. The title solid–solution Zintl compounds were prepared from the elements as single crystals using a tin flux for compositions x = 0, 1, 2, and 3. Eu11Cd6Sb12–xAsx (x \u3c 3) crystallize isostructurally in the centrosymmetric monoclinic space group C2/m (no. 12, Z = 2) as the Sr11Cd6Sb12 structure type (Pearson symbol mC58). Efforts to make the As compositions for x exceeding ∼3 resulted in structures other than the Sr11Cd6Sb12 structure type. Single-crystal X-ray diffraction indicates that As does not randomly substitute for Sb in the structure but is site specific for each composition. The amount of As determined by structural refinement was verified by electron microprobe analysis. Electronic structures and energies calculated for various model structures of Eu11Cd6Sb10As2 (x = 2) indicated that the preferred As substitution pattern involves a mixture of three of the six pnicogen sites in the asymmetric unit. In addition, As substitution at the Pn4 site opens an energy gap at the Fermi level, whereas substitution at the other five pnicogen sites remains semimetallic with a pseudo gap. Thermoelectric properties of these compounds were measured on hot-pressed, fully densified pellets. Samples show exceptionally low lattice thermal conductivities from room temperature to 775 K: 0.78–0.49 W/mK for x = 0; 0.72–0.53 W/mK for x = 1; and 0.70–0.56 W/mK for x = 2. Eu11Cd6Sb12 shows a high p-type Seebeck coefficient (from +118 to 153 μ V/K) but also high electrical resistivity (6.8 to 12.8 mΩ·cm). The value of zT reaches 0.23 at 774 K. The properties of Eu11Cd6Sb12–xAsx are interpreted in discussion with the As site substitution

Enhancement of average thermoelectric figure of merit by increasing the grain-size of Mg_(3.2)Sb_(1.5)Bi_(0.49)Te_(0.01)

Zintl compound n-type Mg_3(Sb,Bi)_2 was recently found to exhibit excellent thermoelectric figure of merit zT (∼1.5 at around 700 K). To improve the thermoelectric performance in the whole temperature range of operation from room temperature to 720 K, we investigated how the grain size of sintered samples influences electronic and thermal transport. By increasing the average grain size from 1.0 μm to 7.8 μm, the Hall mobility below 500 K was significantly improved, possibly due to suppression of grain boundary scattering. We also confirmed that the thermal conductivity did not change by increasing the grain size. Consequently, the sample with larger grains exhibited enhanced average zT. The calculated efficiency of thermoelectric power generation reaches 14.5% (ΔT = 420 K), which is quite high for a polycrystalline pristine material

Airway management in cardiac arrest -- not a question of choice but of quality?

This study presented an innovative method in order to estimate training required for skilful and successful intubations during ED cardiac arrests. Video reviews were taken from a system that routinely records ED staff during cardiac arrests and as these recordings are already part of everyday clinical practice, it is likely that there is minimal Hawthorne effect. Cardiac arrest research often reiterates the fact that the basics should be done well. It is commendable that intubations by the residents in this observational study resulted in a modest mean delay in chest compressions of only 8.6 seconds for the intubation attempt. However, nearly a third of intubation attempts were unsuccessful at the first attempt, and there were 11 oesophageal intubations (albeit they were all recognised) in the 93 patients that were included